The present invention relates to a community antenna television (CATV) communication system in which data communication is performed between a center and subscribers' terminal.
In a CATV system of this type, as shown in FIG. 1, a TV signal from a center is received at a subscriber port from a trunk through a branding device 100 and sent to a TV set 108 through a terminal unit 101. A downward data signal multiplexed onto the TV signal is separated from the latter through a filter 102 in the terminal unit 101, demodulated by a demodulator 105, and supplied to a controller 106 in which judgement is made as to whether a selection channel supplied from a remote controller 107 is a permitted one or not in accordance with an address of the downward data signal. Thereafter, the controller 106 causes a TV tuner 103 to operate through a PLL circuit 104 so that a TV signal input to the TV set 108 is selected.
In this case, a data communication signal for controlling reception is normally supplied from the center by a polling system. In this system, the controller 106 must respond to the data communication signal with the highest priority. Data entered through a key operation by the remote controller 107 is sent out from the terminal unit 101 to the center during polling.
Here, a problem arises in that many functions and fast response times in order to respond to the polling from the center side are required for the controller in each terminal. Therefore, the cost of each terminal is considerably high. Further, there is a disadvantage that the terminals must have individual addresses because polling is performed between the center and each terminal, and hence the time taken for polling becomes longer as the number of the terminals increases. Moreover, there is a further problem in that wire tapping reception may be performed by merely copying the design of any terminal.
There has been proposed a distribution unit system in which channel-selection control for the terminals is performed at a distribution portion. In a controller used in the distribution unit system, however, a function of performing data communication with the center, a function of electronically selecting a channel and controlling the channel selection, and a function of performing data communication with the remote controller must be provided, and it is necessary to respond to each data communication request generated at random. Therefore, not only is the complex controller exceedingly expensive in view of the construction thereof, but also there is a possibility that the terminal may be copied or damaged for the purpose of wire tapping reception because a tuner for selecting a TV signal and a controller for performing reception control are provided in the terminal of each subscriber port.